1. Field of the Invention
The present invention generally relates to a method for forming a poly-silicon film and, more particularly, to a method using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length.
2. Description of the Prior Art
In semiconductor manufacturing, amorphous silicon (a-Si) thin-film transistors (TFTs) are now widely used in the liquid crystal display (LCD) industry because a-Si films can be deposited on a glass substrate at low temperatures. However, the carrier mobility in an a-Si film is much lower than that in a poly-silicon (p-Si) film, so that conventional a-Si TFT-LCDs exhibit a relatively slow response time that limits their suitability for large area LCD devices. Accordingly, there have been lots of reports on converting low-temperature grown a-Si films into p-Si films using laser annealing.
Presently, p-Si films are used in advanced electronic devices such as solar cells, LCDs and organic light-emitting devices (OLEDs). The quality of a p-Si film depends on the size of the crystal grains that form the p-Si film. It is thus the greatest challenge to manufacture p-Si films having large grains with high throughput.
FIG. 1A is a conventional system for forming a p-Si film using sequential lateral solidification (SLS). The system comprises: a laser generator 11 for generating a laser beam 12 and a mask 13 disposed in a traveling path of the laser beam 12. The mask has a plurality of transparent regions 13a and a plurality of opaque regions 13b. Each of the plurality of transparent regions 13a is a stripe region with a width W. The laser beam 12 passing through the transparent regions 13a irradiates an a-Si film 15 on the substrate 14 in back of the mask 13 so as to melt the a-Si film 15 in the stripe regions 15a with a width W. As the laser beam 12 is removed, the melted a-Si film 15 in the stripe regions 15a starts to solidify and re-crystallize to form laterally grown silicon grains. Primary grain boundaries 16 parallel to a long side of the stripe regions 15a are thus formed at the center of the stripe regions 15a and a p-Si film is formed to have crystal grains with a grain length equal to a half of the width W, as shown in 1B.
In order to enhance the throughput, U.S. Pat. No. 6,908,835 discloses a method for forming a poly-silicon film using sequential lateral solidification with two laser irradiations. In U.S. Pat. No. 6,908,835, a mask is used to pattern the laser beam and thus control the grain length, as shown in FIG. 2A and FIG. 2C.
In FIG. 2A, the mask 20 comprises a plurality of first stripe-shaped transparent regions 21 and a plurality of second stripe-shaped transparent regions 22 so that an a-Si film (not shown) on a substrate (not shown) in back of the mask 20 undergoes two laser irradiations while moving relatively to the mask 20 along X-axis. In FIG. 2B, it is given that each of the first and the second transparent regions 21 and 22 has a width W. The spacing between two adjacent first transparent regions 21 and between two adjacent second transparent regions 22 is S. An offset width OS appears between the first transparent regions and the second transparent regions, where OS≧½ W. Therefore, the distance λ between a first primary grain boundary (corresponding to a central line 211 in the first transparent regions 21) obtained after SLS using the first laser irradiation and a second primary grain boundary (corresponding to a central line 221 in the first transparent regions 22) obtained after SLS using the second laser irradiation is λ=(W+S)/2.
In practical cases, however, the system for forming a p-Si film in FIG. 1A can further comprise a projection lens apparatus (not shown) disposed on the traveling path of the laser beam 12 between the substrate 14 and the mask 13. Given that the projection lens apparatus has an amplification factor of N, the grown p-Si film has crystal grains that have a grain length of λ/N. For example, if W=27.5 μm, S=7.5 μm and N=5, the grain length of the p-Si film is λ/N=[(W+S)/2]/5=3.5 μm, as shown in FIG. 2C.
In order to obtain a larger grain length, U.S. Pat. No. 6,726,768 discloses a method for forming a poly-silicon film using sequential lateral solidification with multiple laser irradiations. In U.S. Pat. No. 6,726,768, a mask is used to pattern the laser beam and thus control the grain length, as shown in FIG. 3. In FIG. 3, the mask 30 comprises a plurality of first transparent regions 31, a plurality of second transparent regions 32, a plurality of third transparent regions 33, a plurality of fourth transparent regions 34 and a plurality of fifth transparent regions 35, so that an a-Si film (not shown) on a substrate (not shown) in back of the mask 30 undergoes multiple laser irradiations while moving relatively to the mask 30 along X-axis. Even though a larger grain length of crystal grains may be obtained using the method disclosed in U.S. Pat. No. 6,726,768, it takes longer time and results in low throughput.
Therefore, there exists a need in providing a method for forming a poly-silicon film, using sequential lateral solidification (SLS) with two laser irradiations using a mask for patterning the laser beam so as to increase the grain length.